STATE OF THE ART AND FUTURE DEVELOPMENTS IN NATURAL GAS ENGINE TECHNOLOGIES

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1 Proceedings of DEER 2003: Diesel Engine Emissions Reduction Newport, Rhode Island, August 2003 STATE OF THE ART AND FUTURE DEVELOPMENTS IN NATURAL GAS ENGINE TECHNOLOGIES Mark Dunn, Cummins Westport Inc. ABSTRACT Current, state of the art natural gas engines provide the lowest emission commercial technology for use in mediumheavy duty vehicles. NOx emission levels are 25 to 50% lower than state of the art diesel engines and PM levels are 90% lower than non-filter equipped diesels. Yet, in common with diesel engines, natural gas engines are challenged to become even cleaner and more efficient to meet environmental and end-user demands. Cummins Westport isdeveloping two streams of technologies to achieve these goals for medium-heavy and heavy-heavy duty applications. For medium-heavy duty applications, lowest possible emissions are sought on SI engines without significant increase in complexity and with improvements in efficiency and BMEP. The selected path builds on the capabilities of the CWI Plus technology and recent diesel engine advances in NOx controls, providing potential to reduce emissions to 2010 values in an accelerated manner and without the use of Selective Catalytic Reduction or NOx Storage and Reduction technology. For heavy-heavy duty applications where high torque and fuel economy are of prime concern, the Westport-Cycle technology is in field trial. This technology incorporates High Pressure Direct Injection (HPDI ) of natural gas with a diesel pilot ignition source. Both fuels are delivered through a single, dual common rail injector. The operating cycle is entirely unthrottled and maintains the high compression ratio of a diesel engine. As a result of burning 95% natural gas rather than diesel fuel, NOx emissions are halved and PM is reduced by around 70%. High levels of EGR can be applied while maintaining high combustion efficiency, resulting in extremely low NOx potential. Some recent studies have indicated that DPF-equipped diesels emit less nanoparticles than some natural gas vehicles [1]. It must be understood that the ultrafine particles emitted from SI natural gas engines are generally accepted to consist predominantly of VOCs [2], and that lubricating oil is a major contributor. Fitting an oxidation catalyst to the natural gas engine leads to a reduction in nanoparticles emissions in comparison to engines without aftertreatment [2,3,4]. In 2001, the Cummins Westport Plus technology was introduced with the C Gas Plus engine, a popular choice for transit bus applications. This incorporates drive by wire, fully integrated, closed loop electronic controls and a standard oxidation catalyst for all applications. The B Gas Plus and the B Propane Plus engines, with application in shuttle and school buses were launched in 2002 and The gas-specific oxidation catalyst operates in concert with an optimized ring-pack and liner combination to reduce total particulate mass below 0.01g/bhphr, combat ultrafine particles and control VOC emissions. INTRODUCTION Cummins Westport Inc., a joint venture between Cummins Inc. and Westport Innovations Inc. was formed in 2001 to develop and bring to market a line of low emission, high performance gaseous fueled engines for heavy-duty vehicles. This paper outlines the technology incorporated in today s products, which are considered to represent the state of the art In addition, developments under way to improve customer value and the emissions profile are discussed.. 1

2 COMMERCIAL PRODUCTS THE STATE OF THE ART The commercial offering of Cummins Westport consists of a range of spark ignited lean burn engines based on the established 5.9 liter ISB and 8.3 liter ISC diesel engine platforms. Engines sold into North American and European markets are based on the Plus Technology architecture. Central to this architecture is a highly capable, integrated electronic control system which eliminates the separate electronic governor control module found in previous generations. This control system uses drive-by-wire throttle plate control, knock sensing and a wide-range lambda sensor to provide a wide range of fuel compatibility. An oxidation catalyst is standard in all Plus technology engines. such as heating value and density. Parameters important for emissions and combustion vary also. This include the methane number (a measure of the fuels propensity to knock) and the overall hydrogen to carbon ratio, which influences levels of carbon dioxide emissions. Figure 1 shows the range of higher heating value and methane number found in global sources of natural gas. In general, gases with elevated Higher Heating Value (expressed in volumetric terms) contain larger concentrations on ethane and propane with correspondingly lower methane and sometimes, inert content. The high ethane and propane content reduces the methane number, increasing the likelihood that an engine will suffer from knock. CUMMINS WESTPORT C GAS PLUS The C Gas Plus engine family is rated at up to 280bhp, 850ft-lbs, finding applications in 40 transit buses, refuse collection vehicles and heavy duty trucks to around 35,000lbs Gross Vehicle Weight (GVW). The engine family was launched in July Table 1 gives certification emissions values for the C Gas Plus. NOx and PM emissions are 1.5 and 0.01g/bhp-hr respectively over both the US FTP transient test and Supplemental Emissions Test (SET). NOx NMHC CO PM HCHO Transient Test SET Test N/A A number of vehicle comparisons have been undertaken on the C Gas Plus, both in transit applications [3,4,5] and heavyduty trucks [6]. These highlight the emission benefit available with the engine. Key features of the C Gas Plus include: Robust oxygen sensor Engine back pressure compensation Fuel supply pressure monitoring Knock sensing and control Intake manifold temperature monitoring and protection Supply voltage monitoring Adaptive learn. FUEL COMPATIBILITY The composition of natural gas road vehicle fuel is found to be variable, especially when compared to the well-controlled diesel fuels available in North America. Composition can have geographical variability (generally related to gas source) as well as seasonal variability, which is often related to management of gas supply by the local utility. Typically natural gas consists of mostly methane with much smaller concentrations of heavier hydrocarbons such as ethane, propane and butane. Often, natural gas contains inert compounds such as carbon dioxide and nitrogen. This variability influences parameters important for fuel metering Figure 1 Worldwide Methane Number Analysis for Natural Gas. In order to extend worldwide fuels compatibility of the C Gas Plus engine, a compression ratio of 10:1 is used, along with the advanced controls incorporated in the Plus technology architecture. IMPACT OF THE OXIDATION CATALYST Natural gas is an inherently ultra-low sulfur fuel with typical concentrations of 10ppm. This allows the use of highly active oxidation catalysts without formation of sulfate-derived particulate encountered in diesel engines at current fuel sulfur levels. It is well known that in diesel engines, oxidation catalysts can significantly reduce volatile compounds in the exhaust gas. This includes compounds often classed as toxic [7]. The same is true for natural gas engines, therefore, an oxidation catalyst is a standard component of all Plus Technology engines. These provide effective control of particulate mass, non-methane hydrocarbons, aldehydes and carbon monoxide. Oxidation catalysts are also found to significantly reduce ultra-fine nanoparticles [3,4]. This class of particle is normally considered to consist of mostly volatile organic compounds [2]. By eliminating heavier gas-phase hydrocarbons, the nucleation of particles at the lower temperature conditions that occur after dilution in the ambient environment is mitigated. Figure 2 2

3 shows that on a lean burn natural gas engine, the use of an oxidation catalyst reduces concentrations of ultra-fine nanoparticles (<100nm) by approximately 90%. Approx 90% reduction in <100nm particle concentration Figure 2 - Impact of an Oxidation Catalyst on Ultrafine Nanoparticles [4] LUBRICATING OIL CONTROL AND PARTICULATE EMISSIONS The oxygen-rich, premixed combustion process employed in lean burn natural gas engines results in very low particulate formation rates. When the general combustion conditions favor low particulate formation, the contribution of lubricating oil derived material to the overall particulate emission level becomes significant. Lean burn natural gas engines have different in-cylinder conditions when compared to the diesel engines upon which they are often based. The combination of low peak pressures and sub-ambient minimum pressures at light load conditions results in unique requirements for the oil control system. As natural gas engines have developed over multiple generations oil control has significantly improved. Figure 3 demonstrates that oil consumption is 90% lower at idle in the 3 rd generation when compared to the 1 st, both in the green and run-in condition. Oil Consumption Referenced To First Generation 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Green Run-in 1st Generation 2nd Generation 3rd Generation Figure 3 - Evolution of Lubricating Oil Control This improvement in oil control in later generations propagates to lower particulate emissions with and without an oxidation catalyst. Figure 4 shows particulate emissions over the certification cycle for 1 st and 2 nd generation engines with and without an oxidation catalyst. Comparison of 1 st generation emissions without catalyst to 2 nd generation with catalyst indicates a 90% reduction in total particulate mass. Also, the level of emission in the 2 nd generation without a catalyst is only double that of the 1 st generation with a catalyst. Clearly, the level of oil control development and the presence or not of an oxidation catalyst must be considered when comparing the ultra-fine nanoparticle production characteristics of any natural gas engine to other low-emission technologies. Normalized PM Emission 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Without Catalyst 1st Generation 2nd Generation With Catalyst Figure 4 - Impact of Oil Control and Oxidation Catalyst on Particulate Emissions NEAR TERM DEVELOPMENTS To date, natural gas engines have been generally applied lighter vehicles at or below 40,000lbs GVWR. However, heavier vehicles such as 60 transit buses and heavier Class 8 trucks operate in non-attainment areas where lower emission vehicles can make a positive impact. Cummins Westport Inc. is addressing this requirement by developing higher output heavy duty engines. In the near term, the Plus technology will be applied to the 8.9liter ISL platform, providing up to 320bhp and 950ft-lbs. In addition, Cummins Westport is developing the Westport- Cycle ISX G engine based on the ISX 02, 15 liter platform with cooled EGR, offering the same ratings as comparable diesel products. The Westport-Cycle utilizes late cycle High Pressure Direct Injection (HPDI) of natural gas to achieve a high BMEP, high efficiency combustion approach. Engines using this process retain the Diesel cycle, providing high low-speed torque and efficiency. In the case of the ISX G engine, a diesel fuel pilot injection provides the ignition source. Natural gas is injected at high pressure following ignition of the diesel fuel and during the main combustion phase. For heavy-duty automotive applications, natural gas is derived from pressurized LNG. 3

4 BSCO2 as compared to Diesel (%) Diesel EGR HPDI EGR (Approx. Half Diesel NOx Level) ESC Test Mode 21% Lower GHG 25% Lower CO2 CH 4 Contribution Figure 5 - HPDI Injection Process The combustion approach is highly EGR tolerant, allowing NOx to be reduced by around 50% when compared to the base diesel platform without modifying the air handling or EGR system. Importantly, particulate emissions are reduced by around 70% compared to diesel-fueled operation. Brake Thermal Efficiency 45% 40% 35% 30% 25% 20% Thermal Efficiency Over ESC Zone Composite NOx level 1-1.5g/bhp-hr Lean Burn SI HPDI EGR BMEP [bar] Figure 6 - Brake Thermal Efficiency Comparison, HPDI EGR versus Lean Burn SI A brake thermal efficiency comparison of the HPDI EGR and Lean Burn SI is given in Figure 6. This shows that at a composite NOx level of 1-1.5g/bhp-hr, the HPDI EGR engine is significantly more efficient over the entire operating map. Compared to diesel fueled operation, Green House Gas (GHG) emissions are reduced by 21% in CO 2 equivalents due to the lower carbon content of natural gas and diesel-equivalent fuel efficiency. Figure 7 - Modal Comparison of Green-House-Gas Emissions, Natural Gas vs. Diesel-Fueled Operation Figure 7 shows a modal comparison of GHG emissions over modes 2-13 of the Engine Steady-state Cycle (ESC) for natural gas and diesel fueling. In the natural gas case, the composite NOx emissions are 50% of the diesel case. Under natural gas fueling the engine has consistently lower CO 2 emissions at every mode at between 70-80% of the diesel case. The methane emissions resulting from the combustion process are low and without catalytic treatment only reduce the GHG benefit from 25% to 21%. In addition to developing advanced combustion processes for natural gas, Cummins Westport is investigating the potential of other gaseous fuels including hydrogen. Natural gas is often considered a practical step in the move towards a hydrogen fueled vehicle fleet. It is well known that natural gas can act as a carrier varying quantities of hydrogen. The fuel blend can be stored and delivered by a common fuel system. Range limitations associated with pure hydrogen operation are much less severe with hydrogen / natural gas blends due to the higher volumetric energy density of the gas. Addition of hydrogen to natural gas has been found to extend the lean operating limit allowing NOx emissions to be significantly reduced without loss in efficiency or increase in other emissions. Cummins Westport B Gas Plus engines operating on 20mol% hydrogen are currently under evaluation at SunLine Transit. This blend contains 7% hydrogen on an energy bases and 3% on a mass basis. 4

5 HCNG emissions normalized vs. CNG NOx AVL 8-Mode Comparison NMHC CH4 Figure 8 - Emissions Comparison with 20mol% Hydrogen / Natural Gas Blend Figure 8 shows a steady state emissions comparison for the B Gas Plus at this blend strength. NOx emissions are reduced by 50% with equivalent BSFC (note that this is corrected for the heating value of the blend). CO CO2 BSFC 2007/10 TECHNOLOGY DEVELOPMENTS The challenge for natural gas engines to reach NOx 0.2g/bhp-hr could be considered as more urgent than their diesel counterparts. Whilst the standard has a 50% phase-in period for CARB and EPA heavy-duty truck and EPA urban bus, CARB have retained 100% phase in of the standard for urban bus applications. This requirement for an important natural gas engine market is driving the pace of development of ultra-low NOx natural gas technologies. In 2007, when diesel engines are expected to achieve 1-1.2g/bhp-hr NOx (certified with use of averaging, banking and trading provisions), a significant proportion of natural gas engines could be expected to reach the 0.2g/bhp-hr standard. To serve this need, Cummins Westport have a strong technology development program supported by government funding agencies such as the National Renewable Energy Laboratory (NREL), South Coast Air Quality Management District (SCAQMD), California Energy Commission (CEC), Technology Partnerships Canada (TPC), and Natural Resources Canada (NRCan). The flexibility of natural gas combustion processes and leverage of diesel technology (such as cooled EGR) into this arena are expected to facilitate improved operating efficiency as 2010 approaches. The low emissions demonstrated to date with heavy-duty natural gas engines has focused mainly on highly diluted, low temperature combustion in homogenous and stratified modes. Dilution is achieved with excess air (lean burn) or with inert gases (EGR operation) to provide large NOx reduction while mitigating efficient impacts. Cummins Westport expects to continue this trend with high levels of incylinder NOx control coupled with aftertreatment as required. Today s commercial products meet the 2007 particulate standard via use of oxidation catalysts a trend that is expected to be continued. One example of the potential for ultra-low NOx levels by the application of high levels of EGR to HPDI is given in Figure 9. BSNOx (g/bhp-hr) Heat Release Rate Low EGR Medium EGR High EGR Constant SOC Crank angle NOx g/bhp-hr AVL AVL Mode Mode 7 (High (High speed, speed, 70% 70% power) power) 50% Net Heat Release (Degrees ATDC) Figure 9 - Ultra-low NOx via Application of High Levels of EGR As higher EGR rates are used, the NOx emission and combustion phasing becomes de-coupled, showing potential for high efficiency with 2010 NOx levels. The heat release duration is only marginally extended by increased EGR rates and continues to resemble normal diesel combustion at extremely low NOx emission rates. NOx Storage and Reduction (NSR) systems are one of the technologies under consideration for diesel engines as 2010 approaches. This strategy can be equally well applied to natural gas engines and in some aspects is less challenging than the diesel-based approach: With natural gas comes widespread availability of low cost, ultra-low sulfur fuel (10ppm with CNG, zero ppm with LNG and ~1ppm fuel Sulfur with HPDI LNG/ULSD). To date, Cummins Westport has been working with single bed systems demonstrating regeneration & desulphation with natural gas as a reductant. Tests have been indicated the possibility to achieve 90% conversion efficiency with 2.5% fuel penalty (de-greened condition). The low fuel penalty is partly possible due to low feed gas NOx levels but also results in the high level of flexibility in operating strategy possible with natural gas. Although fuel sulfur levels are low, desulphation is periodically required and long-term deterioration in system NOx conversion levels remains a concern. 5

6 ACKNOWLEDGMENTS The author wishes to acknowledge the efforts of engineering teams at Cummins, Westport and Cummins Westport in achieving the results reported within this paper. In addition, the continued support of our funding partners, US Department of Energy, National Renewable Energy Laboratory (NREL), South Coast Air Quality Management District (SCAQMD), California Energy Commission (CEC), Technology Partnerships Canada (TPC), and Natural Resources Canada (NRCan) is gratefully recognized. Figure 10 - NOx Storage and Reduction System Under Test Spark ignited engines can also benefit from the application of cooled EGR, the basic elements of which have been developed for diesel products. In particular, operation with a three-waycatalyst offers potential for very low NOx levels without out recourse to complex aftertreatment or a negative impact on thermal efficiency and BMEP. NATURAL GAS ENGINE OUTLOOK The outlook for natural gas engines is very positive. Product capability and breadth is continually expanding with the integration of the latest control and combustion technologies. Today s products offer very low emissions and technical evolution will continue throughout the decade. Multiple technologies and development paths exist for natural gas engines to attain 2010 standards. The flexibility of natural gas as a combustion engine fuel contributes to the potential to meet aggressive emissions targets and improve operational efficiency. As a result Cummins Westport expects to be able offer commercial products and significantly surpass mandatory standards through to REFERENCES 1. Holmen, B. et al., 2001 ARB s Study of Emissions from Late-model Diesel and CNG Heavy-duty Transit Buses. 5th International Conference on Nanoparticle Measurements, 2001, Zurich. 2. Andersson, J & Wedekind, B, DETR/SMMT/CONCAWE Particulate Research Programme, Summary Report. Ricardo Report DP01/ Report of Partial Results: Emissions From Two Oxidation Catalyst Equipped CNG, CARB, August Holmen B, Ayala, A. Oxidation Catalyst Effects on Natural Gas Transit Bus Ultrafine Particle Emissions. 6th International ETH Conference on Nanoparticle Measurements August 19-21, 2002, Zurich. 5. US Department of Energy, Energy Efficiency and Renewable Energy, Evaluating the Emission Reduction Benefits of WMATA Natural Gas Buses, NREL/FS Kamel, M et al., An Emission and Performance Comparison of the Natural Gas Cummins Westport Inc. C-Gas Plus Versus Diesel in Heavy-Duty Trucks., SAE Demonstration of Advanced Emission Control Technologies Enabling Diesel Powered Heavy Duty Engines To Achieve Low Emission Levels, MECA 6